The invention relates to crystallogenesis and more particularly to the manufacturing of crystals comprising a plurality of elements wherein the proportion of at least one of the elements is continuously variable, whereby the lattice parameter value of those crystals presents a gradient. Such crystals will be simply called hereinafter gradient crystals.
Gradient crystals are for example usable as monochromators for small wavelength radiations such as neutrons or X-rays.
The idea of using gradient crystals as radiation monochromators appears for example in German patent application 1816542 filed on Dec. 23, 1968 in the name of Institut Max von Laue-Paul Langevin, Grenoble. In this application, the advantage of crystals such as Si.sub.x Ge.sub.1-x (x designating a value comprised between 0 and 1) is emphasized.
Accordingly, for more than twenty years, the advantage of manufacturing gradient crystals has been sensed. However, it has not been possible to industrially manufacture such crystals which are real single crystals, are large enough, and have a composition variation range sufficient for providing a satisfactory lattice gradient. Indeed, one aims to obtain crystals having a size greater than some millimeters, for example a length in the gradient direction of about 10 mm.
Two types of crystal growth processes are known: the processes starting from a liquid phase and the processes starting from gazeous phase. Those two crystal growth families have been known for a long time. Generally, the processes using a liquid phase are used for obtaining large size crystals while the gaseous phase processes, such as the gaseous epitaxies are used for manufacturing thin layer crystals, for example in the field of microelectronics.
During those last twenty years, the approach for manufacturing gradient crystals was based on the liquid phase processes and more particularly on pulling processes wherein, during the pulling, the composition of the liquid phase is continuously varied. Those processes have not given satisfactory results. In particular, for manufacturing gradient crystals of the type Si.sub.x Ge.sub.1-x, those processes have provided single crystals only if the composition variation of germanium with respect to silicium was not higher than some percentage units.
Accordingly, an object of the invention is to provide a general process for manufacturing gradient single crystals which results in a relatively rapid manufacturing while providing a large composition range and a substantial thickness.